Various types of touch screen monitors have been developed to facilitate user interaction with graphical user interfaces (GUIs). Touch screens are gaining popularity for numerous applications, including point-of-information kiosks, vending, electronic catalogs, in-store locators, corporate training, gaming, lottery, and amusement, multimedia marketing, banking/financial transactions, ticket sales, interactive education, multimedia demos, museum displays, and the like. A touch screen generally employs one of four types of touch technologies: capacitive, resistive, infrared, and surface acoustic wave (SAW). In general, capacitive and resistive touch technologies both rely on overlays, whereas infrared and SAW configurations typically do not require overlays.
By way of illustration, an analog resistive screen is formed of a sandwich of Mylar and plastic or glass separated by substantially transparent elastic spacers. The inside surfaces of the sandwich are coated with a uniform transparent thin film, such as a conductive coating. In operation, a voltage is alternately applied along horizontal and vertical axes of the screen. When a user depresses the Mylar overlay so that its conductive layer contacts the energized layer, the resulting voltage is sensed and transmitted to a controller that converts the signal to an indication of touch location.
In a capacitive type of touch screen, a glass panel is coated with a conductive coating that is fused into the glass. The coating is connected to electrodes located at edges of the screen. Each electrode is connected to an oscillator circuit. When a user touches the screen, the body capacitance of the user causes a change in the impedance of the screen. The impedance change causes the oscillator frequencies to vary, and the frequency differentials are converted into a corresponding X-Y coordinate.
The IR technology employs an array of infrared (IR) light emitting diode (LED)/photodetector pairs mounted in a frame. In operation, the LED/photodetector array is continuously and sequentially scanned horizontally and then vertically. When a user touches the display breaking one or more of the light beams, the X-Y position of the touch can be transmitted to a controller or host computer. In order to increase the maximum resolution of an IR touch screen to approximately double the number of LED/photodetector pairs, an interpolation technique can be employed. Using interpolation, when an odd number of beams is broken along either axis, the X or Y coordinate of the center beam is transmitted, but when an even number of beams is broken, the coordinates of the interpolated beam are calculated and transmitted to the host computer.
Today, most commercial acoustic touch screen systems employ surface acoustic waves (SAWs) as the acoustic mode propagating in a faceplate, although other modes can be used, such as horizontally or transversely polarized shear waves. In one particular type of SAW touch screen system (sometimes referred to as the Adler system), the presence and location of a finger or stylus on a faceplate is determined based on disruption of one or more SAWs propagating on the screen. This method propagates a SAW on one side of the faceplate in a beam that is near an edge of the screen. A series of reflectors, which can be gratings, are located along the edge of the screen, each of which reflects a portion of the energy across the faceplate at about 90° relative to the edge and the direction in which the beam is traveling. The density of reflectors is varied so that the amplitude of the signals propagating across the faceplate is nearly constant. A second set of reflectors are located at the opposite edge of the screen, which reflect the SAW into a beam propagating substantially parallel to the edge. The spacing of the reflectors is chosen so that the multiple beams propagating across the faceplate cover substantially the entire surface. The arrival time of the various beams at the receiving transducer increases monotonically as the total path length increases.
By way of illustration, when a finger or other object contacts the screen, it interrupts the SAW causing a decrease in the received amplitude corresponding to the position of the finger or object. As a result, this approach bases detection on the blockage or disruption of a transmitted SAW and on the spacing of the multiplicity of beams for spatial resolution.